1. Field of the Invention
The present invention relates to an information processing apparatus utilizing the technology of scanning tunnel microscope.
2. Related Background Art
Recently, the use for memory material constitutes the nucleus of electronics industry of electronic equipment such as computer and its peripheral device, video and audio disk equipment, and development of such material is being conducted actively. Conventionally, magnetic and semiconductor memories consisting of magnetic and semiconductive materials, have been utilized principally, but inexpensive optical memories capable of high density recording, consisting of an organic thin film, for example, of organic pigments or photopolymer, are being introduced as a result of progress in the laser technology.
On the other hand, the recent development of scanning tunnel microscope (STM) capable of directly observing the electron structure on or in the vicinity of the material surface [G. Binnig et al., Helvetica Physica Acta, 55, 726 (1982)] has enabled the observation of a real spatial image of monocrystalline or non-crystalllne material with a high resolving power. Wider applications are expected for said microscope, as it can effect the measurement with a low electric power and with no damage to the specimen by the current, and as it can be applied to various substances not only under high vacuum but also in atmospheric pressure or in solution.
STM is based on a phenomenon that a tunnelling current is generated when a metal probe is brought to a distance of about 1 nm to a conductive specimen, with a voltage applied therebetween. Since said tunnelling current is extremely sensitive to and exponentially varies with the said distance, the real spatial shape of the specimen surface can be obtained with a resolving power of about 0.1 nm along the surface, by scanning the specimen surface with the probe while maintaining a constant tunnelling current therebetween.
According to application of the above-explained principle of the STM, it is possible to perform the recording and reproduction of information in the atomic (sub-nanometer) order. So, there have been proposed various information processing apparatus for recording, reproduction and/or erasure of information with an ultra high density, based on said principle. For example, Japanese Patent Application Laid-open No. 61-80536 discloses the information recording by elimination of atomic particles adhered on the recording medium with an electron beam. Also, there is proposed information recording method by physical or magnetic breakdown of the surface of the recording medium with a laser beam, an electron beam or a particle beam.
Also, U.S. Pat. No. 4,575,822 discloses information recording by a charge injection with the tunnelling current into a dielectric layer formed on the surface of the recording medium. In these cases, the recording and reproduction of information are conducted by STM. Furthermore, Japanese Patent Application Laid-open Nos. 63-161552 and 63-161553 propose the information recording and reproduction with STM, utilizing a substance with memory effect on the voltage-current switching characteristic, such as organic compounds of a .pi.-electron system or chalcogenide compounds, as a thin film recording layer. Such method enables a large-capacity recording of 10.sup.12 bit/cm.sup.2 in a recording bit size of 10 nm.
In such information processing apparatus, it is essential to effect the scanning operation with a constant average tunnelling current between the probe and the recording medium, and control methods therefor are proposed, for example, in Japanese Patent Application Laid-open Nos. 1-53363 and 1-133239.
FIG. 1 shows a conventional control circuit for the distance between the probe and the recording medium, wherein the tunnelling current flowing between a probe electrode 1 and a recording medium 2 is amplified by an amplifier 3. A band component extracted by a low-pass filter 4 is supplied through a phase compensation circuit 5, an error amplifier 6 and a sample hold circuit 7, and is used for feedback control on the operating voltage of a cylindrical piezoelectric actuator 8 connected to the probe electrode 1, so that the average tunnelling current is maintained constant. On the other hand, a band component extracted by a high-pass filter 9 is supplied to a signal processing circuit 10 thereby reproducing the information. Upon input of recording signal, the sample hold circuit 7 is utilized to maintain a constant output signal for the probe electrode 1.
The above-explained control is also possible in a circuit shown in FIG. 2. The current flowing between a probe electrode 1 and a recording medium 2 is supplied through a current amplifier 3, a logarithmic amplifier 11, a low-pass filter 4, an amplifier 12, a phase compensation circuit 5, an error amplifier 6 and a sample hold circuit 7, and is used for feedback control on the operating voltage of a cylindrical piezoelectric actuator 8. The information is reproduced from the output of a low-pass filter 4 in a signal processing circuit 10. Upon input of a recording signal, the probe electrode 1 is operated in a similar manner as in the circuit shown in FIG. 1.
In such information processing apparatus, the probe electrode 1 has to be positioned upon access to the recording medium 2. In such high-density memory, there is generally employed an access method in which plural edges (tracks) for positioning are formed on the recording medium 2 in advance as shown in FIG. 4, and after detecting a desired edge, the probe electrode 1 is moved to an access position (information column) along the edge. Upon access in direction B, there is frequently conducted movement from a track to another track. Such movement between tracks is executed by giving an offset value to the driving voltage of a scanning mechanism for the probe electrode 1, or by moving a rough moving mechanism itself.
In the conventional information processing apparatus, since the probe electrode 1 is moved along the plane of the recording medium 2 with the average tunnelling current maintained substantially constant as explained above, tip of the probe electrode 1 follows a broken-lined trajectory shown in FIG. 3 when it crosses a recessed track groove T in an access operation.
Since the tip of the probe electrode 1 is generally made very sharp, it has to be prevented from contact with the recording medium 2. For this reason, in the above-explained conventional example, the probe electrode 1 is scanned with speed enabling feedback control, in order to avoid collision of the probe electrode 1 with the recording medium 2 when said probe electrode 1 comes out of the recess on the recording medium 2. The fact that the scanning speed is limited by the ability of feedback control, gives rise to a limitation in application fields requiring a high access speed.